Patent GB-687561-A (GB)
Title: Improvements in or relating to cathode ray tubes
IPC: H01J29/54
Filed: 1947-01-24
Abstract: 687,561. Cathode-ray tubes. RADIO CORPORATION OF AMERICA. Jan. 24, 1947 [Jan. 24, 1946; Jan. 30, 1946], Nos. 2357/47 and 2358/47. Class 39(i) [Also in Group XXXV] A cathode ray tube assembly comprises means for causing electrons and ions in the electron beam to follow a path at an angle to the axis of the tube, means for producing a first magnetic field to cause the electrons to follow a path directed towards the tube axis so that the beam is substantially freed from ions and means for producing a second magnetic field whereby the electrons are directed substantially axially of the tube. In the construction shown in Fig. 1 the cathode ray tube 10 comprises an indirectly heated cathode 16, a control electrode 20, and an electrostatic lens 34 constituted by a first anode 22 and a second anode 24 connected by a spring element 28 to a conducting wall coating 26 of graphite connected to a lead 30. The spring element 28 may be omitted and the second anode 24 connected to a separate source of potential which is positive with respect to the potential of the first anode 22. Means may be provided for varying the potential difference between the anodes 22 and 24. Adjacent surfaces of the first anode 22 and the second anode 24 are substantially parallel and are disposed at an angle other than 90 degrees with their axes and with the axis of the tube 10. Connections of the various electrodes to voltage sources can be as in Specification 360,654, [Group XXXVI]. The cathode ray beam 36 after leaving the lens 34 passes between the deflecting yoke 32 including a pair of horizontal, or line, deflection coils and a pair of vertical, or frame, deflection coils and then strikes a fluorescent screen. During its passage through the lens 34 the beam is deflected along the path 38, is subjected to the action of a first magnetic field A transverse to the axis of the tube to deflect the electrons along the path 42, the ions striking the electrode 24 at the point 40, and is then subjected to the action of a second magnetic field B parallel to the first magnetic field but of opposite polarity and lesser intensity to deflect the electrons along the path 46. Fig. 3 illustrates one form of electromagnet assembly around the neck of the tube for producing the magnetic fields A and B comprising a pair of pole pieces 52 for producing the magnetic field A, coils 56 of the order of 250 ampere turns, for a tube operating at six kilovolts, and core segments 58 to which are secured the pole pieces 54 for producing the magnetic field B. The intensity ratio of the fields A and B can be varied by varying the width of the air gap 60 between the ends of the segments 58. A pair of openings 62 in the segments 58 permit the electromagnet assembly to be bolted to a supporting member. A second form of electromagnet assembly is shown in Fig. 5 and comprises a first electromagnet 76 including a core 77 on which is wound a coil 78 and pole pieces 79 secured to the ends of the core by rivets 80, the pole pieces being fixed by rivets 81 to non-magnetic supporting members 82 formed of material known under the Registered Trade Mark "Bakelite". The pole pieces 79 and the supporting members 82 are provided with arcuate recesses 83 for encircling the neck of the cathode ray tube. An arcuate slot 84 is formed in each supporting member 82 coaxial with the arcuate recesses 83 to receive bolts or screws permitting rotational adjustment of the pole pieces 79 about the axis of the tube. A second electromagnet 86 is similar to the first 76, the coil 88 being wound in a direction opposite to that of the coil 78 or being so connected in a direct current circuit as to produce a magnetic field opposite to that produced by the coil 78. The electromagnets 76 and 86 may be independently rotated about the axis of the tube and independently adjusted axially along the tube. In the construction shown in Fig. 6 a pair of oppositely disposed angle shaped pole pieces 96 are soldered to the outer surface of the first anode 66 so as to lie adjacent the faces of the external pole pieces 79 to tend to sharpen the field pattern within the first anode 66. Curves are given showing the relationship between the strength of the current in the coils 78 and 88 and the spacing L of the poles 79 from the end of the control electrode 65 for spacings M between the pole pieces of ¥" and 1". Coil 78 can be wound with 2,000 turns and coil 88 with 300 turns and the faces of the pole pieces 79 and 89 can be approximately #" wide. The voltage on the first anode 66 can be 250 volts and on the second anode 67 can be 6 kilovolts. A curve is also given showing the relationship between coil current and the second anode voltage. A further construction of the electromagnet assembly is shown in Figs. 9 and 10 and comprises a pair of U or horse-shoe shaped core sections 97 and 98 of magnetic material and coils 99 and 100 wound midway on the core sections 97 and 98 respectively. The core sections 97 and 98 are held in spaced-apart parallel relation by means of two pairs of internally threaded tubular spacers, one pair of which is shown at 101 and 102 in Fig. 10. A short screw 103 engages the threaded spacer 101 and passes through an opening in the core section 98 and a longer screw 104 engages the threaded spacer 102 and passes through an opening in the core section 97. The screw 104 extends into the open end of the threaded spacer 101 and holds in place a pair of outwardly-flared disc-shaped spring metal members 105 and 106 for positioning the electromagnet assembly on the neck of the cathode ray tube. The ratio of the currents through the coils 78 and 88 of Fig. 5 or 99 and 100 of Fig. 9 can be selectively controlled by the potentiometer arrangement 107, 110 and 111 shown in Fig. 11. One of the potentiometers 110 and 111 may be omitted.